Ceramic honeycomb structures are known in the prior art. Such structures generally comprise a plurality of webs that define a plurality of cells, and a skin surrounding the plurality of webs and having an inner surface or inner wall integrally connected to peripheral ends of the webs. Such ceramic honeycomb structures are currently favored for use as ceramic catalyst substrates for automobile exhaust systems. To maximize contact between the exhaust and the catalyst coated over the webs, high cell densities on the order of 400-1200 cells per square inch are favored. However, to minimize the pressure drop with such a structure, thin webs on the order of 0.001 to 0.005 inches are typical. During manufacture, both the plurality of webs and the skin are simultaneously extruded such that sharp-cornered or filleted joints are formed between the peripheral ends of the webs, and the inner wall of the skin. The skin is typically made considerably thicker than the webs so that the extrusion which ultimately becomes the final ceramic honeycomb structure will have sufficient mechanical strength to endure the handling and cutting procedures required to render the raw extrusion into a final product.
Unfortunately, the sharp or slightly filleted interfaces between the thin webs forming the plurality of cells and the inner wall of the skin tend to concentrate mechanical stresses in these areas when mechanical forces are applied to the exterior of the skin during the manufacturing process. Additionally, the applicant has observed that the substantially greater thickness of the skin can create thermally-induced stresses at these interfaces, as the greater thickness of the skin inherently gives it a different coefficient of thermal expansion (CTE) than the substantially thinner cell walls, even though both walls are formed from the same ceramic material. Finally, the inventor has observed that during the extrusion process, these same differences in thickness between the webs and the skin creates differences in flow rate of the plasticized ceramic material as it is squeezed through the extrusion die that forms the honeycomb structure. It is theorized that these differences in the velocity of material flow weakens the joints between the peripheries of the outer webs and the inner wall of the skins.
Previous approaches to solving structural problems relating to honeycomb strength and/or the web-skin interface include a thickening of the webs in the vicinity of the periphery of the body (as shown in U.S. Pat. No. 4,233,351) and a heavier rounding or filleting of the corners of the joints between these components of the structure (as shown in U.S. Pat. No. 5,952,079 and U.S. Pat. No. 6,060,148). However, the thickening of the webs interferes with gas flow around the periphery of the structure, and the rounding of the corners of the joints does nothing to reduce stresses arising during the forming of the webs and skins which are thought to contribute to structural defects in the manufactured articles. Thus problems such as skin flaking and chipping from the outside of the fired honeycombs during handling, or even localized separations of the skin from the plurality of webs occurring during drying or firing, continue to be observed in some honeycomb designs.
Clearly, what is needed is a ceramic honeycomb structure having improved joints between the webs, and the skin which more effectively resists skin flaking and chipping, skin separation, and thermal damage in use. Such improved joints should not result in a significant thickening of either the webs or the skin such that larger thermally-induced stresses between these components would be created due to differences in CTE. Ideally, such joints would allow the skin to be made thinner so that the CTE differentials between the skin and webs could be reduced. Finally, it would be desirable if such stress-reduced joints could be easily produced by way of conventional extrusion techniques which would reduce or eliminate any differences in flow speed in the ceramic material in the joint interface area such that the resulting joints would be stronger.